Xfinity VOIP sounds like bad connection - caller voice in and out

In all calls, the voice on the other side comes in and out - sounds similar to being on a mobile with poor cellular connectivity. This happened last year and rebooting modem would correct it for a while - it eventually fixed itself as Comcast / Xfinity knew there was a prob they were working on. Now it is happening on every call and rebooting does not correct the issue.